The overall objective of the proposed work is the molecular genetic analysis of pathways of development and morphogenesis through the isolation and characterization of conditional-lethal mutations in the genes involved. The key element is the development of new genetic methods for isolation of mutants specifically affecting pathways of interest, the determination of which gene products interact with each other. In order to accomplish this objective, we are combining approaches involving a prokaryotic system (the Salmonella phage P22) and a eukaryotic microorganism (the yeast Saccharomyces cerevisiae).